This invention relates to the use of bisulfite-blocked polyisocyanates as tanning agents.
Tanning converts animal skins into leather by crosslinking the collagen. One of the most important characteristics of leather is an elevated shrinkage temperature compared with untanned skins, that is, an enhanced resistance to hot water and a white (non-transparent, non-pigmented) appearance after drying.
The type of tanning that is currently becoming increasingly dominant is chrome tanning, in which chromium (III) compounds, under the influence of OH.sup.- ions, provide a crosslinking effect by forming covalent bonds with the carboxyl groups of the collagen. In contrast, the hydrogen bonds to the amide groups of the collagen that can be obtained with polyfunctional vegetable tanning agents are much weaker, which also results in a shrinkage temperature that is only moderately increased. Aliphatic aldehydes, such as glutaraldehyde, which cause crosslinking via the primary amino groups of the collagen, have also been recommended as tanning agents (U.S. Pat. No. 2,941,859). However, the resultant aldimines can undergo a reversible reaction in the presence of water to again form aldehyde and amine.
It has not been possible in practice to use aliphatic diisocyanates, such as hexamethylene diisocyanate (German Patentschrifi 72,981), for toxicological reasons.
The use of bisulfite-blocked aliphatic, cycloaliphatic, or aromatic diisocyanates such as hexamethylene diisocyanate, isophorone diisocyanate, and toluene diisocyanate as tanning agents, as recommended in U.S. Pat. Nos. 2,923,594 and 4,413,997, produces light, non-fading leather, but the tanning liquors are not pH stable.
It has surprisingly now been found that the use of bisulfite-blocked polyisocyanates containing polyethylene oxide groups provides unforeseen advantages relating to commercial application.